So just in case someone finds this thread five years from now, it was the starter! Bought a second hand one and it works beautifully, better than the old one ever was. I wasn't able to open the old one as there was one screw that was seized on and my mate's impact driver bit was too big. In the end I wasted a bunch of money because I was impatient - I bought a new battery ($80), reg/rec ($20), and starter solenoid ($40). Thanks to everyone for all your help!
I'm sure happy to hear my theory of something inside the cases tightening up was wrong for your sake. I like happy endings...